A wavefront is defined as a surface of

The correct answer is Option (2) → Constant phase

A wavefront is the locus of all points of the medium that oscillate in the same phase at a given instant. The direction of propagation of the wave is perpendicular to the wavefront.

NCERT (2026-27):
Huygens Principle – “… Such a locus of points, which oscillate in phase is called a wavefront; thus a wavefront is defined as a surface of constant phase.”